The invention relates to a method for recognizing a water passage or a high water level for a vehicle, which has, as a drive, an internal combustion engine having an exhaust train, in which the functionality of an emission control system is monitored or controlled by at least one exhaust sensor and the exhaust sensor is operated at least intermittently at high temperatures and has thermal shock susceptibility by design.
The invention further relates to a device, in particular a control and evaluation unit for carrying out the method according to the invention.
According to the prior art, exhaust sensors, such as, e.g., lambda probes, particle sensors or nitrogen oxide sensors, are based on ceramic sensor elements which are heated at least intermittently during operation.
Particle sensors (PM) are used, for example, today to monitor the carbon-particulate emissions of internal combustion engines or for the on-board diagnostics (OBD), for example for the functional monitoring of particle filters, e.g. a diesel particle filter (DPF). Such a resistive particle sensor is described in the German patent specification DE 101 33 384 A1. The particle sensor is constructed from two interlocking, comb-shaped electrodes which are at least partially covered by a catch sleeve. If particles from a gas stream are deposited on the particle sensor, this then leads to an analyzable change in the impedance of the particle sensor, from which an inference can be made about the amount of accumulated particles and thus about the amount of the particles being carried in the exhaust gas.
If the particle sensor is fully loaded, the accumulated particles are burned in a regeneration phase with the aid of a heating element in the particle sensor. To this end, the ceramic of the sensor element is heated to high temperatures, typically to >600° C. In this regeneration phase, the sensor element reacts sensitively to large local changes in temperature or, respectively, to a thermal shock, as said shock can arise from the impact of water or water drops. Such a thermal shock can lead to cracks in the sensor element. A sensor regeneration is then only requested by the engine control device if, according to a heat quantity calculation in the engine control device, water can no longer be present at the sensor installation position.
In addition, it is essential to prevent that the particle sensor is subjected to water when the temperature of the sensor element of such exhaust sensors is greater than a specified threshold temperature, typically approximately 200° C. For this reason, the operation takes place when heating the exhaust sensors at a temperature >200° C., in particular after a cold start as long as condensation can still be present in the exhaust train of the internal combustion engine, first after a certain amount of time, in which it can be assumed that all of the water has either evaporated or has been removed in the form of droplets from the exhaust train by means of gas discharges during a driving operation. This point in time is typically denoted as the dew point end (DPE) and depends on many conditions. That is why this point in time has to be determined for each vehicle type in accordance with the application. The operation of the exhaust gas sensors at temperatures >200° C. is allowed as long as there is no condensation of water in the exhaust train in the region of the sensor installation point as a result of cooling.
By severely subjecting the exhaust system to water from the outside, e.g. when traversing water or when putting a boat into water on a slip ramp, water can penetrate into the exhaust system, whereby the exhaust system can be severely cooled down. Depending on the configuration of the exhaust system, this cooling down of said system cannot be recognized by the engine control device but can present a risk of thermal shock at a sensor element of the exhaust sensors. This case is not covered to this point when dating the dew point end because the exhaust sensors up to now typically are primarily installed in the exhaust train of the internal combustion engine so as to be in close proximity to the engine. Hence, it is unlikely that water would come in contact with the exhaust gas sensor when traversing water. By using new exhaust sensors, such as particularly particle or nitrogen oxide sensors, which, in a system-dependent way, are located far to the back of the exhaust train, it is necessary to expand the functionality of the dew point end-determination. To this end, a water passage has to recognized, which up until now has not been the case.
It is furthermore important to recognize a water passage early on in order to have time to take suitable measures to protect the sensor. The sensor elements to be protected can be operated at temperatures to over 800° C. so that a cooling of said sensor elements under 300° C. can take several seconds to minutes.
In the WIPO patent application WO 2012/080439 A1, a system is described for indicating water passages of a vehicle, said system comprising an emitter/receiver sensor having an oscillating membrane and a control unit, wherein the sensor is designed as an ultrasonic sensor. A method for recognizing water by means of this sensor is furthermore described. This sensor can additionally be used as a distance sensor or parking sensor. The system can furthermore comprise a sensor for determining the depth of the water. The initiation of protective measures for protecting the emission control system or, respectively, the exhaust sensors installed therein upon reaching critical water levels is however not disclosed.
It is therefore the aim of the invention to see in advance a water crossing, respectively a water passage, which is associated with an abrupt cooling of the exhaust train by means of surrounding water, in particular downstream of a particle filter, and thereupon to be able to take anticipatory measures in order to protect the ceramic based exhaust sensors, which were mentioned above, in this region from a thermal shock.
It is furthermore the aim of the invention to provide a corresponding device for carrying out the method, in particular a control and evaluation unit.